Plastic containers are widely used for foods and beverages, and also for non-food materials. Poly(ethylene terephthalate) (PET) is used to make many of these containers because of its appearance (optical clarity), ease of blow molding, chemical and thermal stability, and its price. PET is generally fabricated into bottles by blow molding processes, and generally by stretch blow molding.
In stretch blow molding, PET is first shaped by injection molding into a thick-walled preformed parison (a "preform"), which typically is in the shape of a tube with a threaded opening at the top. The parison may be cooled and then used later in a subsequent step, or the process may be carried out in one machine with cooling just to the stretch blow molding temperature. In the stretch blow molding step, the parison is heated to a high enough temperature in a mold to allow shaping, but not so hot that it crystallizes or melts (i.e., just above the glass transition temperature Tg, typically about 90.degree. to 160.degree. C.). The parison is expanded to fill the mold by rapidly stretching it mechanically in the axial direction (e.g., by using a mandrel) while simultaneously forcing air under pressure into the heated parison to expand it radially. PET is typically modified for blow molding with a small amount of comonomer, usually 1,4-cyclohexanedimethanol or isophthalic acid, which increases the width of the temperature window for successful blow molding to about 9.degree. C. The comonomer is necessary because of the need to have a wider temperature window, and also to decrease the rate of stress induced crystallization. At the same time, the comonomer may have the undesirable effect of lowering the glass transition temperature and reducing the crystallinity of PET. Stretch blow molding of PET, and blow molding processes in general, are well known in the art. Reviews are widely available, as for example, "Blow Molding" by C. Irwin in Encyclopedia of Polymer Science And Engineering, Second Edition, Vol. 2, John Wiley and Sons, New York, 1985, pp. 447-478.
This technology is widely used, but there are still improvements that need to be made. First, a wider temperature window for blow molding would greatly enhance the process. Second, a material that can be filled with liquid or solid foods at higher temperatures than are currently used would significantly expand the usefulness of the bottles by allowing packaging at elevated temperatures up to 88.degree. C., and preferably even higher, as is necessary for pasteurized foods, beverages and syrups that are too viscous to transfer without being heated. The maximum fill temperature for bottle-resin grades of PET is generally about 60.degree. C. to 65.degree. C. It is generally believed that a resin with a higher Tg would be better for this purpose.
PET bottles are currently modified for hot fill applications by annealing the bottles in the hot mold for a few seconds immediately after stretch blow molding. This allows PET, which is oriented during stretch blow molding, to partially crystallize before the bottle is demolded and cooled. This can be done in such a way that the crystallinity is sufficiently low and the crystallite size sufficiently small that the bottle is still transparent. The crystallites in PET bottles apparently stabilize the bottles so that they can be exposed to hot liquids, which are at a temperature of up to about 88.degree. C., during the hot filling process without deforming. The annealing step significantly lengthens the time required to make a bottle, resulting in reduced productivity and higher costs. Therefore, a polymer which has a high Tg and low crystallinity is desirable for forming hot and cold fill containers.
The diol 1,4:3,6-dianhydro-D-sorbitol, referred to hereinafter as isosorbide, the structure of which is illustrated below, is readily made from renewable resources, such as sugars and starches. For example, isosorbide can be made from D-glucose by hydrogenation followed by acid-catalyzed dehydration. ##STR1##
Isosorbide has been incorporated as a monomer into polyesters that also include terephthaloyl moieties. See, for example, R. Storbeck et al, Makromol. Chem., Vol. 194, pp. 53-64 (1993); R. Storbeck et al, Polymer, Vol. 34, p. 5003 (1993). However, it is generally believed that secondary alcohols such as isosorbide have poor reactivity and are sensitive to acid-catalyzed reactions. See, for example, D. Braun et al., J. Prakt.Chem., Vol. 334, pp. 298-310 (1992). As a result of the poor reactivity, polyesters made with an isosorbide monomer and esters of terephthalic acid are expected to have a relatively low molecular weight. Ballauff et al, Polyesters (Derived from Renewable Sources), Polymeric Materials Encyclopedia, Vol. 8, p. 5892 (1996).
Copolymers containing isosorbide moieties, ethylene glycol moieties, and terephthaloyl moieties have been reported only rarely. A copolymer containing these three moieties, in which the mole ratio of ethylene glycol to isosorbide was about 90:10, was reported in published German Patent Application No. 1,263,981 (1968). The polymer was used as a minor component (about 10%) of a blend with polypropylene to improve the dyeability of polypropylene fiber. It was made by melt polymerization of dimethyl terephthalate, ethylene glycol, and isosorbide, but the conditions, which were described only in general terms in the publication, would not have given a polymer having a high molecular weight.
Copolymers of these same three monomers were described again recently, where it was observed that the glass transition temperature Tg of the copolymer increases with isosorbide monomer content up to about 200.degree. C. for the isosorbide terephthalate homopolymer. The polymer samples were made by reacting terephthaloyl dichloride in solution with the diol monomers. This method yielded a copolymer with a molecular weight that is apparently higher than was obtained in the German Patent Application described above but still relatively low when compared against other polyester polymers and copolymers. Further, these polymers were made by solution polymerization and were thus free of diethylene glycol moieties as a product of polymerization. See R. Storbeck, Dissertation, Universitat Karlsruhe (1994); R. Storbeck, et al., J. Appl. Polymer Science, Vol. 59, pp. 1199-1202 (1996).
U.S. Pat. No. 4,418,174 describes a process for the preparation of polyesters useful as raw materials in the production of aqueous stoving lacquers. The polyesters are prepared with an alcohol and an acid. One of the many preferred alcohols is dianhydrosorbitol. However, the average molecular weight of the polyesters is from 1,000 to 10,000, and no polyester actually containing a dianhydrosorbitol moiety was made.
U.S. Pat. No. 5,179,143 describes a process for the preparation of compression molded materials. Also, described therein are hydroxyl containing polyesters. These hydroxyl containing polyesters are listed to include polyhydric alcohols, including 1,4:3,6-dianhydrosorbitol. Again, however, the highest molecular weights reported are relatively low, i.e., 400 to 10,000, and no polyester actually containing the 1,4:3,6-dianhydrosorbitol moiety was made.
Published PCT Applications WO 97/14739 and WO 96/25449 describe cholesteric and nematic liquid crystalline polyesters that include isosorbide moieties as monomer units. Such polyesters have relatively low molecular weights and are not isotropic.